Process of producing anodic oxide films on aluminum

ABSTRACT

AN ANODIC COATING ON ALUMINUM AND ITS ALLOYS CAN BE OBTAINED BY IMMERSING THE ALUMINUM AS AN ANODE IN AN AQUEOUS ELECTROLYTE CONTAINING BORIC/HYDROCARBOXYLIC CHELATE ACID ANIONS TO WHICH SULFURIC ACID OR WATER SOLUBLE SULFATE OR BISULFATE SALTS ARE ADDED, AND PASSING AN ELECTRICAL CURRENT OF SUITABLE VOLTAGE AND CURRENT DENSITY THROUGH THE ELECTROLYTE SOLUTION FOR A PREDETERMINED TIME.

United States Patent 3,682,790 PROCESS OF PRODUCING ANODIC OXIDE FILMS ON ALUMINUM Richard D. Orth, Jack G. Surendranath, and Diptiman Chakravarti, Seattle, Wash, assignors to Innova Corporation, Seattle, Wash. No Drawing. Filed July 29, 196?, Ser. No. 854,341 Int. Cl. C2311 9/02 U.S. Cl. 204-58 19 Claims ABSTRACT OF THE DISCLOSURE An anodic coating on aluminum and its alloys can be obtained by immersing the aluminum as an anode in an aqueous electrolyte containing boric/hydrocarboxylic chelate acid anions to which sulfuric acid or water soluble sulfate or bisulfate salts are added, and passing an electrical current of suitable voltage and current density through the electrolyte solution for a predetermined time.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a method of electrolytically coating aluminum or its alloys using an aqueous electrolyte solution.

PRIOR ART RELATING TO THE DISCLOSURE The production of anodic oxide films on aluminum and its alloys has been accomplished by anodizing in aqueous electrolyte solutions of sulfuric acid, oxalic acid or sulfuric acid and oxalic acid mixtures. Anodizing solutions of boric acid with or without sulfuric acids are known to give anodic coatings. The anodic coatings produced, however, are barrier layer or high resistance coatings of no more than about one micron in thickness. Barrier layer coatings are not useful in architectural applications.

Similarly, anodizing solutions of oxalic acid with sulfuric acid are known to give anodic coatings. For example, see US. Patent 3,252,875.

Processes for producing anodic films on various metals are discussed in detail in Young, Anodic Oxide Films, Academic Press, 1961.

SUMMARY OF THE INVENTION This invention has as one of its objects the production of anodic oxide coatings on aluminum and its alloys.

A further object of this invention is a method of producing corrosion resistant anodic oxide films on aluminum and its alloys using an electrolyte solution comprising boric/hydroxycarboxylic chelate acid anions to which a minor amount of sulfuric acid or water soluble sulphate or bisulfate salts are added.

The electrolyte containing boric/hydroxycarboxylic chelate acid anions to which sulfuric acid or water solu- Patented Aug. 8, 1972 ble sulphate or bisulfate salts are added in small amounts results, on passing an electric current through the electrolyte, in colored porous or low-resistance anodic coatings which grow at constant voltage to the desired thickness. For practical reasons, a thickness up to 1.5 mils is sufficient. Kinetics for the coating using the anodizing electrolyte of this invention are not well understood; however, when boric acid is mixed with the hydroxycarboxylic acids set forth herein the resulting hydrogen ion concentration of the electrolyte solution is much higher because of the formation of a complex ortho boric chelate of boric acid and the hydroxycarboxylic acid. The chelate acid anions in the electrolyte solution, from whatever source obtained, make up the major portion of the electrolyte solution.

Hydroxycarboxylic acids useful in the anodizing solution of this invention include those having more than 2 carbon atoms, specifically alpha hydroxycarboxylic acids such as malic acid, tartaric acid, mandelic acid, -2-hydroxybutanoic acid, and Z-hydroxy, Z-methylpropanoic acid; beta hydroxycarboxylic acids such as 2-hydroxypropanoic acid, Z-methyl, Z-hydroxypropanoic acid, citric acid; polyhydroxy carboxylic acids such as glyceric acid, 2,3,4-trihydroxybntanoic acid and gluconic acid; and gamma hydroxycarboxylic acids. Mixtures of the hydroxycarboxylic 'aicds can also be used.-

Sulfuricacid or water soluble sulfate or bisulfate salts, or mixtures thereof, are added to the electrolyte solution in relatively small quantities, i.e. as little as 0.001 gm. per liter calculated as sulphate. The concentration of sulfuric acid or sulfate or bisulfate salts can beas high as 5.0 gms./liter if desired, although this is not necessary. Water soluble sulphate salts such as alkali metal sulfates or bisulfates or ammonium sulphate or bisulfate can be used.

The concentration of boric acid used to form the chelate in the anodizing solution may range from 0.2 Molar up to the solubility limit of boric acid in aqueous solutions and preferably from 0.2 to 1.0 Molar.

The concentration of hydroxycarboxylic acid used to form the chelate in the anodizing solution ranges preferably from 0.2 to 2.0 Molar.

Aluminum or its alloys are immersed in the anodizing electrolyte solution previously described. Stainless steel or other suitable materials are used as the cathode. The anodizing process is carried out using DC current.

The electrolyte solution described may be employed over a wide range of operating conditions (voltage, current density, temperature). A DC voltage of up to 150 volts is normally applied to maintain a predetermined current density. Normally a current density ranging from 12 to 36 amps/ sq. ft. is used. The current density is maintained constant until a predetermined peak voltage is reached, at which time the current density is allowed to fall. The peak voltage may be the final voltage. The temperature of the anodizing electrolyte solution is usually maintained between and F.

Time of treatment depends upon current density, thickness of coating desired and color of the coating desired. The colors of the anodic film produced by the above process are various shades of grey and bronze.

Table I illustrates the operating conditions employed in anodizing an aluminum 6063 alloy using anodizing plea, and 0.001 to 5.0 grams per liter sulfuric acid, and

passing a direct electrical current of up to 150 volts and a current density of from 12 to 36 amps. per sq. ft. through the electrolyte solution between the anode electrolyte solutions of several different hydroxycarbox-- and cathode at an electrolyte solution temperature of ylic acids at different concentrations. The thickness and 60 to 90 F. to obtain a bronze-colored coating on color of the anodic films are described. the aluminum anode.

TABLE I 51 1i? Boric Hydroxy Voltage, Current, designaacid, carboxylic Grams] Grarns/ volts Temp., amps] Tune, Thickness, tion g./l. acid liter S04 hter D.C. F. sq.it. mins. mils Color 1.-.. 6063 Alloy 19.4 Citric acid mono- 60.0 NazS0 1.8-3.0 0-100 70 24 30 0.5-0.8 Medium to dark hydrate. bronze. 2.-. .-do 30.0 do 60.0 H2804 0. 7-2.0 0-120 70 24 30 0.4-0.6 Liglhgntzoe dark (d)tartaric acid HzSOl 1.0 0-120 70 24 0.3 Medium hronze; (dl)malic aeid---.-. 60.0 H2804 0 1-0.7 0-120 70 24 30 0.2 Light brown. 27.7 do 60.0 H4804 0 8-1.0 0-80 70 24 30 0.4-0.0 Ll%1l1:ntz0e dark Gluconic acid 60.0 H2S04 0.5 0-55 70 24 13 0.2 Mediumhronze. (dDmandelic acid.-. 60.0 H2804 0.6-1.2 0-120 70 24 30 0.2 Gray. --.do 60.0 H4804 1 32.7 0-52 70 24 30 0.4-0.9 Light to dark brown.

The embodiments of the invention in which an exam 11. An aqueous anodizing electrolyte consisting essensive property or privilege is claimed are defined as fol- 25 tially of boric/hydroxy carboxylic chelate acid anions lows: formed in equilibrium with boric acid and a hydroxycar- 1. A method of producing a low-resistance porous, boxylic acid capable of forming a complex lboric chelate bronze-colored anodic coating on aluminum comprising: With a resulting increase in the hydrogen ion concentraproviding at least one cathode, tion of the electrolyte and at least 0.001 gram per liter immersing the aluminum as anode in an aqueous elecsulfate ions from sulfuric acid, water soluble sulfate or trolyte consisting essentially of boric hydroxycarbisulfate salts. boxylic chelate acid anions formed in equilibrium 12. The electrolyte solution of claim 11 wherein the with boric acid and a hydroxycarboxylic acid capable hydroxycarboxlic acid is one selected from the group conof forming a complex boric chelate with a resulting sisting of citric acid, tartaric acid, malic acid, gluconic increase in the hydrogen ion concentration of the acid and mandelic acid. electrolyte and at least 0.001 gram per liter sulfate 13. The electrolyte solution of claim 11 wherein the ions from sulfuric acid, water soluble sulfate or biconcentration of boric acid ranges from 0.2 Molar up to sulfate salts, and the limit of its solubility in an aqueous solution, the conpassing a direct electrical current through the eleccentration of hydroxycarboxylic acid ranges from 0.2. to trolyte solution between the anode and cathode to 2.0 Molar and the concentration of sulfate ion ranges obtain a colored anodic coating on the aluminum from 0.001 to 5.0 grams per liter. anode. 14. The solution according to claim 11 wherein the hy- 2. The method according to claim 1 wherein the droxycarboxylic acid is citric acid. aqueous electrolyte solution comprises at least 0.2 Molar 15. The solution according to claim 11 wherein the hyboric acid, 0.2 to 2.0 Molar hydroxycarboxylic acid and droxycarboxylic acid is tartaric acid. at least 0.001 gm. per liter sulfate. 16. The solution according to claim 11 wherein the hy- 3. The method according to claim 1 wherein the elecdroxycarboxylic acid is malic acid. trical current is passed through the electrolyte solution at 17. The solution according to claim 11 wherein the hya current density ranging from about 12 to 36 amps./sq. droxycarboxylic acid is gluconic acid. ft. 18. The solution according to claim 11 wherein the hy- 4.The method according to claim 1 wherein the hydrocarboxylic acid is mandelic acid. droxycarboxylic acid is one selected from the group con- 19. Anodized aluminum having a low resistance, sisting of citric acid, tartaric acid, malic acid, gluconic porous, bronze-colored anodic coating produced by the acid and mandelic acid. method of claim 1.

5. The method according to claim 1 wherein the hydrzxyzrboxylic acid is citric acid. Refere e cit d e method according to claim 1 wherein the h droxycarboxylic acid is tartaric acid. y UNITED STATES PATENTS 7. The method according to claim 1 wherein th b 140634413 7/ 1913 Chllbb 204-58 droxycarboxylic acid is malic acid. 60 9884104 3/ 9 1 nk s 204-58 8. The method according to claim 1 wherein the hy- 2,231,086 2/1941 Mime! et a1 2 58 droxycarboxylic acid is gluconic acid. 2,150,395 '9 Nagata 204-68 9. The method according to claim 1 wherein th hy- 2,052,575 9/ 1.936 Lllienfeld 204-58 droxycarboxylic acid is mandelic acid.

10. A method of producing a porous, bronze-colored FOREIGN PATENTS anodic coating on aluminum comprising: 226,697 7/ 1943 S r d 04-58 providing at least one cathode, 37,219 3/ 1930 Switzerland 204-58 immersing the aluminum as anode in an aqueous elec- OTHER REFERENCES trolyte solutlon consisting essentially of from 0.2 70 7 a: to L0 Molar boric acid 02 to 20 M01 a r hydroxy The Siurface Treatment and Fll'llSh-ll'lg of Alummum, carboxylic acid selected from the group consisting of by Mernlck and Pinner, 1964, pp. 365-366, 414, 338. citric acid, tartaric acid, maleic acid, gluconic acid and mandelic acid, the boric acid and hydroxycar- JOHN MACK Pnmary Exammer boxylic acid forming a boric/hydroxycarboxylic com- R. L. ANDREWS, Assistant Examiner 

